This invention relates to rotary drills and drill bits and has been developed in relation to machinedriven rotary and rotary percussive drills for rock drilling although it has other applications.
Drill bits originally designed for use with hand-held rotary drills, have radially extending and axially inclined cutting edges formed by intersecting leading and frontal faces at the tip of the bit, the leading face being that face which is inclined at a small angle to a plane containing the axis of rotation of the bit, and the frontal face being that face which is inclined at a small angle to a plane normal to the axis of rotation of the bit. When such drill bits are power driven by high-thrust, high-torque machines, the drilling is more effective and harder rock can be drilled but failures tend to occur because neither the drill bit nor the associated drilling rod has adequate strength even when of high grade material.
The present invention aims to improve the strength of drills and drill bits for heavy duty and particularly for high torque drive.
Alternative to rotary drilling, there is the technique of percussive drilling, wherein chisel bits or cross bits are used with a machine which withdraws the bit from the work face between each blow, the axial movement being used to index the bit angularly.
The present invention is not concerned with such percussive indexing techniques but it does have important application in rotary drilling machines of high thrust and torque and in which a percussive action is introduced. This percussive action is very different from percussive indexing, in that the bit is not withdrawn from the work face between blows and the relatively fast rotation and high thrust is continuously maintained during drilling. The rotary percussive drilling machine may employ a percussive vibration of high frequency and low energy or of low frequency and high energy. Such machines impose still greater stresses on the drill bit and existing standard drill bits are inadequate.